The invention is a controller for an ink jet cartridge in a plotter or similar device in which the cartridge can travel at various angles of travel with respect to a fixed axis in a plane of a recording medium to record a graphic image thereon. The controller of the invention regulates the printed line density (ink dots per meter) recorded on the recording medium to a desired density by programmably adjusting the ink drop rate of the cartridge with changes in the angle of travel. The invention employs a processor with programmable look-up tables relating various angles of travel to respective ink drop rates for various rapidly selectable printed line densities.
Ink jet printers are well-known in the art, as exemplified by U.S. Pat. No. 4,176,363 to Kasahara and U.S. Pat. No. 4,168,533 to Schwartz. Devices for controlling the print head motion are well-known, as exemplified by U.S. Pat. No. 4,422,781 to Armfield et al. and devices for scaling the image size are disclosed in U.S. Pat. No. 4,517,578 Tazaki. Various techniques for controlling the ink drop rate from the ink cartridge are known. For example, U.S. Pat. No. 4,791,435 to Smith et and U.S. Pat. No. 4,544,931 to Watanabe et al. is close regulating the ink drop rate in accordance with temperature. U.S. Pat. No. 4,524,364 to Bain et al. discloses controlling the ink drop rate in accordance with the delay between emission of the ink drop and its impact upon the medium. U.S. Pat. No. 4,542,384 to Tazaki suggests the feature of switching the print dot density on the medium between a text mode and a graph mode. Coupling actuation of the ink jet cartridge to the velocity of the cartridge in one dimension is disclosed in U.S. Pat. No. 4,436,439 to Koto and U.S. Pat. No. 4,789,874 Majette et al.
None of the foregoing references addresses the problem of maintaining a uniform printed line density recorded by a plotter whose cartridge can move at various angles with respect to the X and Y axes of the planar media to draw shapes and circles, for example. The problem arises because the cartridge travels at a faster velocity with respect to the recording medium for a given cartridge speed along the X axis (for example) as its speed along the Y axis increases. The foregoing references teach sensing velocity in one dimension with a shaft encoder, which of course cannot sense such an increase. Without a compensating mechanism, the printed line density will vary with the trajectory of the cartridge. Lines recorded along a 45 degree angle will have the smallest density while lines lying along the X axis, for example, will have the largest density. The line density of a circle recorded on the medium will be different at different parts of the circle. Thus, the image quality will suffer.
U.S. Pat. No. 4,215,353 to Kaieda et al. discloses an analog feedback control loop for computing the two-dimensional velocity of the ink jet cartridge using both X axis and Y axis velocity sensing and setting the ink jet cartridge pressure to control the printed line width (as distinguished from the line density or ink jet drop rate) to an amount proportional to the two-dimensional velocity of the cartridge.
The analog feedback loop described in the patent to Kaieda et al. suffers from several significant disadvantages. First, it does not control the ink jet drop rate but instead affects the ink jet cartridge pressure, which affects the printed dot size or line width but does not directly determine the printed line density (e.g., number of dots per meter). Thus, the teachings of the Kaieda et al. have nothing to do with maintaining constant line printing density for various angles of travel of the cartridge.
A further disadvantage is that its performance is limited by the response characteristics and stability of the analog feedback loop controlling the ink jet cartridge pressure. The components in the feedback control loop of Kaieda et al., which include a differential amplifier for example, must enable the feedback control loop to respond to rapid changes in direction encountered when recording sharp curves, etc., while enabling the feedback loop to remain stable. Such components are relatively expensive, are dedicated to the feedback control loop and therefore represent an added expense and bulk in the printer. In some cases the recorded image quality may be limited by the characteristics of the feedback control loop.
Yet another disadvantage of the feedback loop of Kaieda et al. is that it does not lend itself to rapid changes in the selected line density, for example to achieve gray scale variations or special effects in the recorded image. Such a change in the selected line density would require changing at least one of the parameters of the feedback loop.
Still another disadvantage of the feedback loop of Kaieda et al. is that it relies upon the cartridge servo X and Y shaft encoders to deduce the two-dimensional velocity of the cartridge. The drawback is that the shaft encoder senses position, and velocity must be inferred by noting the elapsed time between subsequent position counts of the encoder (for example). Thus, there is a delay equal to the elapsed time and the time required for the computation. Moreover, any error by one of the shaft encoders will affect the operation of the feedback loop.
Accordingly, it is an object of the invention to regulate the ink jet drop rate in accordance with the angle of travel of the cartridge across the media so as to maintain the printed line density at a selected density for all angles of travel without requiring a feedback control loop.
It is another object of the invention to regulate the ink jet drop rate in accordance with the angle of travel of the cartridge so as to maintain the printed line density at a selected density for all angles of travel without requiring additional hardware beyond that required in any plotter.
It is a further object of the invention to regulate the ink jet drop rate with respect to a single axis of motion of the cartridge in accordance with the angle of travel with respect to that axis so as to maintain the printed line density at a selected density for all angles of travel.
It is still another object of the invention to regulate the ink jet drop rate in accordance with the angle of travel of the cartridge so as to maintain the printed line density at a selected density for all angles of travel without using the X and Y axis shaft encoders to deduce the angle of travel.
It is a still further object of the invention to regulate the ink jet drop rate in accordance with the angle of travel of the cartridge so as to maintain the printed line density at a target density, while rapidly changing the target density in accordance with gray scale changes or special effects in a digitized image being plotted or in accordance with a selection by the user of a different printed line density.
A further object of the invention is to regulate the ink jet drop rate in accordance with the angle of travel of the cartridge so as to maintain the printed line density at a target density, while rapidly changing the target density in accordance with gray scale changes and while changing the dot size in accordance with half-tone or special effects in a digitized image being plotted or in accordance with selections made by the user.
These and other objects and benefits of the invention will become apparent in the detailed description which follows hereinafter when taken in conjunction with the drawings which accompany it.